The present embodiments relate to a printed circuit board.
In MR tomography, images having a high signal-to-noise ratio (SNR) may be acquired using local coils. The nuclei of the atoms excited to magnetic resonances induce a voltage in the local coil, the induced voltage being amplified using a low-noise (pre)amplifier (LNA). The amplified signal is forwarded to receive electronics. The signal may be forwarded via shielded cables (e.g., coaxial cables).
High-field systems (e.g., magnetic resonance systems, in which the static basic magnetic field is in the region of 3 Tesla or more) are employed even in the case of high-resolution images in order to improve the signal-to-noise ratio further.
More coils may be connected to a magnetic resonance receiving system than there are receive channels present. For this reason, a switching array is installed between the receive antennas and the receive electronics. The switching array routes the currently active local coils onto the receive channels present. This enables more local coils to be connected to the receive electronics than there are receive channels available.
The local coils are positioned in the radio-frequency transmit field of the body coil (BC; e.g., whole-body coil). The body coil is a large coil that encompasses the body of the examination subject (e.g., a human being) and is used to excite the spins in the body. The body coil generates a circularly or elliptically polarized radio-frequency field. Owing to the radio-frequency field, currents are induced on all the conductors located within the radio-frequency field. Resonance effects may be produced (e.g., in the case of relatively long cables) with the result that high currents flow in the shielding. The currents may be so great that the currents may lead to a patient being put at risk due to heating. Even before such hazardous situations occur, the induced currents may also cause secondary radio-frequency fields that destroy the homogeneity of the radio-frequency field generated by the body coil.
A further problem occurs if there is a discontinuity in the shielded cable (e.g., if the shielded cable is soldered onto a printed circuit board or the shielded mode is exited in some other way). In that case, the preamplified receive signals transmitted over the shielded conductors may also cause interference. For example, if the cable is routed in proximity to an antenna, the preamplified signal may feed back into the antenna again. This may result in self-oscillation of the local coil.
Standing wave traps may be used for suppressing shield currents. Various design formats for standing wave traps are known. It is known, for example, to wind a coaxial cable into a coil and to connect two points of the shield to an appropriately dimensioned capacitor. It is also known to cover a coaxial cable with an electrically conductive outer sleeve (e.g., a balun) and connect the balun capacitively to the cable shield. Other types of design are also known.
Prior art standing wave traps have various disadvantages. Thus, for example, the standing wave traps are wound manually and consequently are expensive. Solder joints are also applied manually to the cable shields, which is problematic in manufacturing process terms. Because of the large diameter of coaxial cables and the large bending radius associated therewith, standing wave traps also have high space requirements. The space requirement is problematic, for example, in the case of coils that are intended to be mechanically flexible. Large volumes are used for the standing wave traps (e.g., when a plurality of coaxial cables are to be routed substantially in parallel).
Individual shielded wires may not be brought out from the shield before the end of the cable.